Apparatus for fabricating nanoscale patterns in light curable compositions using an electric field

ABSTRACT

The present invention is directed to an apparatus for patterning a liquid on a substrate, with the apparatus including, a template having a pair of spaced-apart recessions with a protrusion disposed therebetween, with the protrusion being spaced-apart from the substrate a first distance and each of the pair of spaced-apart recessions being spaced-apart from the substrate a second distance, with the second distance being greater than the first distance; and a source of voltage in electrical communication with the template to produce an electric field between the template and the substrate, with a strength of the electrical field being inversely proportional to the first and second distances.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application is a divisional of U.S. patentapplication Ser. No. 09/905,718 filed on May 16, 2001 entitled “Methodand System for Fabricating Nanoscale Patterns in Light CurableCompositions using an Electric Field,” which is incorporated byreference herein.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention generally relates to the area of low cost,high-resolution, high-throughput lithography with the potential to makestructures that are below 100 nm in size.

[0004] 2. Description of the Relevant Art

[0005] Optical lithography techniques are currently used to makemicroelectronic devices. However, these methods are reaching theirlimits in resolution. Sub-micron scale lithography has been a criticalprocess in the microelectronics industry. The use of sub-micron scalelithography allows manufacturers to meet the increased demand forsmaller and more densely packed electronic components on chips. Thefinest structures producible in the microelectronics industry arecurrently on the order of about 0.13 μm. It is expected that in thecoming years, the microelectronics industry will pursue structures thatare smaller than 0.05 μm (50 nm). Further, there are emergingapplications of nanometer scale lithography in the areas ofopto-electronics and magnetic storage. For example, photonic crystalsand high-density patterned magnetic memory of the order of terabytes persquare inch require nanometer scale lithography.

[0006] For making sub-50 nm structures, optical lithography techniquesmay require the use of very short wavelengths of light (for instance13.2 nm). At these short wavelengths, few, if any, materials areoptically transparent and therefore imaging systems typically have to beconstructed using complicated reflective optics [1]. Furthermore,obtaining a light source that has sufficient output intensity at thesewavelengths of light is difficult. Such systems lead to extremelycomplicated equipment and processes that appear to be prohibitivelyexpensive. High-resolution e-beam lithography techniques, though veryprecise, typically are too slow for high-volume commercial applications.

[0007] One of the main challenges with current imprint lithographytechnologies is the need to establish direct contact between thetemplate (master) and the substrate. This may lead to defects, lowprocess yields, and low template life. Additionally, the template inimprint lithography typically is the same size as the eventualstructures on the substrate (1×), as compared to 4× masks typically usedin optical lithography. The cost of preparing the template and the lifeof the template are issues that may make imprint lithographyimpractical. Hence there exists a need for improved lithographytechniques that address the challenges associated with opticallithography, e-beam lithography and imprint lithography for creatingvery high-resolution features.

SUMMARY OF THE INVENTION

[0008] The present invention is directed to an apparatus for patterninga liquid on a substrate, with the apparatus including, a template havinga pair of spaced-apart recessions with a protrusion disposedtherebetween, with the protrusion being spaced-apart from the substratea first distance and each of the pair of spaced-apart recessions beingspaced-apart from the substrate a second distance, with the seconddistance being greater than the first distance; and a source of voltagein electrical communication with the template to produce an electricfield between the template and the substrate, with a strength of theelectrical field being inversely proportional to the first and seconddistances. These and other embodiments are discussed below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIGS. 1A-1E illustrate a version of the imprint lithographyprocess according to the invention;

[0010]FIG. 2 is a process flow diagram showing the sequence of steps ofthe imprint lithography process of FIGS. 1A-1E;

[0011]FIG. 3 is a side view of a template positioned over a substratefor electric field based lithography;

[0012]FIG. 4 is a side view of a process for forming nanoscalestructures using direct contact with a template;

[0013]FIG. 5 is a side view of a process for forming nanoscalestructures using non-direct contact with a template;

[0014]FIG. 6 is a side view of a substrate holder configured to alterthe planarity of the substrate; and

[0015]FIG. 7 is a side view of an apparatus for positioning a templateover a substrate.

DETAILED DESCRIPTION OF THE INVENTION

[0016]FIGS. 1A through 1E illustrate an imprint lithography processaccording to the invention, denoted generally as 10. In FIG. 1A, atemplate 12 is orientated in spaced relation to a substrate 14 so that agap 16 is formed in the space separating template 12 and substrate 14. Asurface 18 of template 12 is treated with a thin layer 20 that lowersthe template surface energy and assists in separation of template 12from substrate 14. The manner of orientation including devices forcontrolling of gap 16 between template 12 and substrate 14 are discussedbelow. Next, in FIG. 1B, gap 16 is filled with a substance 22 thatconforms to the shape of surface 18. Preferably, substance 22 is aliquid so that it fills the space of gap 16 rather easily without theuse of high temperatures and gap 16 can be closed without requiring highpressures.

[0017] A curing agent 24, shown in FIG. 1C, is applied to template 12causing substance 22 to harden and to assume the shape of the spacedefined by gap 16 between template 12 and substrate 14. In this way,desired features 26, shown in FIG. 1D, from template 12 are transferredto the upper surface of substrate 14. A transfer layer 28 is provideddirectly on the upper surface of substrate 14 which facilitates theamplification of features transferred from template 12 onto substrate 14to generate high aspect ratio features.

[0018] In FIG. 1D, template 12 is removed from substrate 14 leaving thedesired features 26 thereon. The separation of template 12 fromsubstrate 14 must be done so that desired features 26 remain intactwithout shearing or tearing from the surface of substrate 14.

[0019] Finally, in FIG. 1E, features 26 transferred from template 12,shown in FIG. 1D, to substrate 14 are amplified in vertical size by theaction of transfer layer 28, as is known in the use of bi-layer resistprocesses. The resulting structure can be further processed to completethe manufacturing process using well-known techniques. FIG. 2 summarizesthe imprint lithography process, denoted generally as 30, of the presentinvention in flow chart form. Initially, at step 32, course orientationof a template and a substrate is performed so that a rough alignment ofthe template and the substrate is achieved. The advantage of courseorientation at step 32 is that it allows pre-calibration in amanufacturing environment where numerous devices are to be manufacturedwith efficiency and with high production yields. For example, where thesubstrate comprises one of many die on a semiconductor wafer, coursealignment (step 32) can be performed once on the first die and appliedto all other dies during a single production run. In this way,production cycle times are reduced and yields are increased.

[0020] Next, at step 34, the spacing between the template and thesubstrate is controlled so that a relatively uniform gap is createdbetween the two layers permitting the type of precise orientationrequired for successful imprinting. The present invention provides adevice and a system for achieving the type of orientation (both courseand fine) required at step 34. At step 36, a liquid is dispensed intothe gap between the template and the substrate. Preferably, the liquidis a UV curable organosilicon solution or other organic liquids thatbecome a solid when exposed to UV light. The fact that a liquid is usedeliminates the need for high temperatures and high pressures associatedwith prior art lithography techniques.

[0021] At step 38, the gap is closed with fine orientation of thetemplate about the substrate and the liquid is cured resulting in ahardening of the liquid into a form having the features of the template.Next, the template is separated from the substrate, step 40, resultingin features from the template being imprinted or transferred onto thesubstrate. Finally, the structure is etched, step 42, using apreliminary etch to remove residual material and a well-known oxygenetching technique to etch the transfer layer.

[0022] As mentioned above, recent imprint lithography techniques with UVcurable liquids [2, 3, 4, 5] and polymers [6] have been described forpreparing nanoscale structures. These techniques may potentially besignificantly lower cost than optical lithography techniques for sub-50nm resolution. Recent research [7, 8] has also investigated thepossibility of applying electric fields and van der Waals attractionsbetween a template that possesses a topography and a substrate thatcontains a polymeric material to form nanoscale structures. Thisresearch has been for systems of polymeric material that may be heatedto temperatures that are slightly above their glass transitiontemperature. These viscous polymeric materials tend to react very slowlyto the electric fields (order of several minutes) making them lessdesirable for commercial applications.

[0023] The embodiments described herein may potentially createlithographic patterned structures quickly (in a time of less than about1 second). The structures may have sizes of tens of nanometers. Thestructures may be created by curing a polymerizable composition (e.g., aspin-coated UV curable liquid) in the presence of electric fields.Curing the polymerizable composition then sets the pattern of structureson the substrate. The pattern may be created by placing a template witha specific nanometer-scale topography at a carefully controllednanoscale distance from the surface of a thin layer of the liquid on asubstrate. If all or a portion of the desired structures are regularlyrepeating patterns (such as an array of dots), the pattern on thetemplate may be considerably larger than the size of the desiredrepeating structures. The template may be formed using direct writee-beam lithography. The template may be used repeatedly in ahigh-throughput process to replicate nanostructures onto substrates. Inone embodiment, the template may be fabricated from a conductingmaterial such as Indium Tin Oxide that is also transparent to UV light.The template fabrication process is similar to that of phase shiftphotomasks for optical lithography; phase shift masks require an etchstep that creates a topography on the template.

[0024] The replication of the pattern on the template may be achieved byapplying an electric field between the template and the substrate.Because the liquid and air (or vacuum) have different dielectricconstants and the electric field varies locally due to the presence ofthe topography of the template, an electrostatic force may be generatedthat attracts regions of the liquid toward the template. At highelectric field strengths, the polymerizable composition may be made toattach to the template and dewet from the substrate at certain points.This polymerizable composition may be hardened in place bypolymerization of the composition. The template may be treated with alow energy self-assembled monolayer film (e.g., a fluorinatedsurfactant) to aid in detachment of the template the polymerizedcomposition.

[0025] It may be possible to control the electric field, the design ofthe topography of the template and the proximity of the template to theliquid surface so as to create a pattern in the polymerizablecomposition that does not come into contact with the surface of thetemplate. This technique may eliminate the need for mechanicalseparation of the template from the polymerized composition. Thistechnique may also eliminate a potential source of defects in thepattern. In the absence of contact, however, the liquid may not formsharp, high-resolution structures that are as well defined as in thecase of contact. This may be addressed by first creating structures inthe polymerizable composition that are partially defined at a givenelectric field. Subsequently, the gap may be increased between thetemplate and substrate while simultaneously increasing the magnitude ofthe electric field to “draw-out” the liquid to form clearly definedstructures without requiring contact.

[0026] The polymerizable composition may be deposited on top of ahard-baked resist material to lead to a bi-layer process. Such abi-layer process allows for the formation of low aspect ratio,high-resolution structures using the electrical fields followed by ananisotropic etch that results in high-aspect ratio, high-resolutionstructures. Such a bi-layer process may also be used to perform a “metallift-off process” to deposit a metal on the substrate such that themetal is left behind after lift-off in the trench areas of theoriginally created structures.

[0027] By using a low viscosity polymerizable composition, the patternformation due to the electric field may be fast (e.g., less than about 1sec.), and the structure may be rapidly cured. Avoiding temperaturevariations in the substrate and the polymerizable composition may alsoavoid undesirable pattern distortion that makes nano-resolutionlayer-to-layer alignment impractical. In addition, as mentioned above,it is possible to quickly form a pattern without contact with thetemplate, thus eliminating defects associated with imprint methods thatrequire direct contact.

[0028]FIG. 3 depicts an embodiment of the template and the substratedesigns. Template 12 may be formed from a material that is transparentto activating light produced by curing agent 24 to allow curing ofsubstance 22, with substance 22 being a polymerizable composition, byexposure to activating light. Forming template 12 from a transparentmaterial may also allow the use of established optical techniques tomeasure gap 16 between template 12 and substrate 14 and to measureoverlay marks to perform overlay alignment and magnification correctionduring formation of the structures. Template 12 may also be thermallyand mechanically stable to provide nano-resolution patterningcapability. Template 12 may also include an electrically conductingmaterial to allow electric fields to be generated at thetemplate-substrate interface.

[0029] In one embodiment, depicted in FIG. 3, a thick blank of fusedsilica has been chosen as the base material for template 12. Indium TinOxide (ITO) may be deposited onto the fused Silica. ITO is transparentto visible and UV light and is a conducting material. ITO may bepatterned using high-resolution e-beam lithography. Thin layer 20 (forexample, a fluorine containing self-assembly monolayer) may be coatedonto template 12 to improve the release characteristics between template12 and substance 22. Substrate 14 may include standard wafer materials,such as Si, GaAs, SiGeC and InP. A UV curable liquid may be used assubstance 22. Substance 22 may be spin coated onto substrate 14. Anoptional transfer layer 28 may be placed between substrate 14 andsubstance 22. Transfer layer 28 may be used for bi-layer process.Transfer layer 28 material properties and thickness may be chosen toallow for the creation of high-aspect ratio structures from low-aspectratio structures created in substance 22. An electric field may begenerated between template 12 and substrate 14 by connecting the ITO toa voltage source.

[0030] In FIGS. 4 and 5, two variants of the above-described process arepresented. In each variant, it is assumed that a desired uniform gap 16may be maintained between template 12 and substrate 14. An electricfield of the desired magnitude may be applied resulting in theattraction of substance 22 towards the raised portions of template 12.In FIG. 4, gap 16 and the field magnitudes are such that substance 22makes direct contact and adheres to template-12. A UV curing process maybe used to harden substance 22 in that configuration. Once thestructures have been formed, template 12 is separated from substrate 14by either increasing gap 16 till the separation is achieved, or byinitiating a peel and pull motion wherein template 12 is peeled awayfrom substrate 14 starting at one edge of template 12. Prior to its use,template 12 is assumed to be treated with thin layer 20 that assists inthe separation step.

[0031] In FIG. 5, gap 16 and the field magnitudes are chosen such thatsubstance 22 achieves a topography that is essentially the same as thatof template 12. This topography may be achieved without making directcontact with template 12. A UV curing process may be used to hardensubstance 22 in that configuration. In both the processes of FIGS. 4 and5, a subsequent etch process may be used to eliminate the residual layerof the UV cured material. A further etch may also be used if transferlayer 28 is present between substance 22 and substrate 14, as shown inFIGS. 4 and 5. As mentioned earlier, transfer layer 28 may be used toobtain a high-aspect ratio structure from a low aspect ratio structurecreated in substance 22.

[0032]FIG. 6 illustrates mechanical devices that may increase theplanarity of the substrate. The template may be formed from high-qualityoptical flats of fused-silica with Indium Tin Oxide deposited on thefused silica. Therefore, the template typically possess extremely highplanarity. The substrates typically have low planarity. Sources ofvariations in the planarity of the substrate include poor finishing ofthe back side of the wafer, the presence of particular contaminantstrapped between the wafer and the wafer chuck, and wafer distortionscaused by thermal processing of the wafer. In one embodiment, thesubstrate may be mounted on a chuck whose top surface shape may bealtered by a large array of piezoelectric actuators. The chuck thicknessmay be such that accurate corrections in surface topography of up to afew microns may be achieved. The substrate may be mounted to the chucksuch that it substantially conforms to the shape of the chuck. Once thesubstrate is loaded on to the chuck, a sensing system (e.g., an opticalsurface topography measurement system) may be used to map the topsurface of the substrate accurately. Once the surface topology is known,the array of piezoelectric actuators may be actuated to rectify thetopography variations such that the upper surface of the substrateexhibits a planarity of less than about lam. Since the template isassumed to be made from an optically flat material, this leads totemplate and substrate that are high quality planar surfaces.

[0033] The mechanical device in FIG. 7 may be used to perform ahigh-resolution gap control at the template-substrate interface. Thisdevice may control two tilting degrees of freedom (about orthogonal axesthat lie on the surface of the template) and the vertical translationdegree of freedom of the template. The magnitude of the gap between thetemplate and the substrate may be measured in real-time. These real-timemeasurements may be used to identify the corrective template motionsrequired about the tilting degrees of freedom and the verticaldisplacement degree of freedom. The three gap measurements may beobtained by using a broadband optical interferometric approach that issimilar to the one used for measuring thicknesses of thin films and thinfilm stacks. This approach of capacitive sensing may also be used formeasuring these three gaps.

[0034] Further modifications and alternative embodiments of variousaspects of the invention will be apparent to those skilled in the art inview of this description. Accordingly, this description is to beconstrued as illustrative only and is for the purpose of teaching thoseskilled in the art the general manner of carrying out the invention. Itis to be understood that the forms of the invention shown and describedherein are to be taken as the presently preferred embodiments. Elementsand materials may be substituted for those illustrated and describedherein, parts and processes may be reversed, and certain features of theinvention may be utilized independently, all as would be apparent to oneskilled in the art after having the benefit of this description of theinvention. Changes may be made in the elements described herein withoutdeparting from the spirit and scope of the invention as described in thefollowing claims.

References

[0035] The following references are specifically incorporated herein byreference:

[0036] 1. “Getting More from Moore's,” Gary Stix, Scientific American,April 2001.

[0037] 2. “Step and Flash Imprint Lithography: An alternative approachto high resolution patterning,” M. Colburn, S. Johnson, M. Stewart, S.Damle, B. J. Choi, T. Bailey, M. Wedlake, T. Michaelson, S. V.Sreenivasan, J. Ekerdt, C. G. Willson, Proc. SPIE Vol.3676, 379-389,1999.

[0038] 3. “Design of Orientation Stages for Step and Flash ImprintLithography,” B. J. Choi, S. Johnson, M. Colburn, S. V. Sreenivasan, C.G. Willson, To appear in J. of Precision Engineering.

[0039] 4. U.S. patent application Ser. No. 09/266,663 entitled “Step andFlash Imprint Lithography” to Grant Willson and Matt Colburn.

[0040] 5. U.S. patent application Ser. No. 09/698,317 entitled “HighPrecision Orientation Alignment and Gap Control Stages for ImprintLithography Processes” to B. J. Choi, S. V. Sreenivasan and SteveJohnson.

[0041] 6. “Large area high density quantized magnetic disks fabricatedusing nanoimprint lithography,” W. Wu, B. Cui, X. Y. Sun, W. Zhang, L.Zhunag, and S. Y. Chou., J. Vac Sci Technol B 16 (6) 3825-3829November-December 1998

[0042] 7. “Lithographically-induced Self-assembly of Periodic PolymerMicropillar Arrays,” S. Y. Chou, L. Zhuang, J Vac Sci Tech B 17 (6),3197-3202, 1999

[0043]8. “Large Area Domain Alignment in Block Copolymer Thin FilmsUsing Electric Fields,” P. Mansky, 1. DeRouchey, J. Mays, M. Pitsikalis,T. Morkved, H. Jaeger and T. Russell, Macromolecules 13,4399 (1998).

What is claimed is:
 1. An apparatus for patterning a liquid on a substrate, said apparatus comprising: a template having a pair of spaced-apart recessions with a protrusion disposed therebetween, with said protrusion being spaced-apart from said substrate a first distance and each of said pair of spaced-apart recessions being spaced-apart from said substrate a second distance, with said second distance being greater than said first distance; and a source of voltage in electrical communication with said template to produce an electric field between said template and said substrate, with a strength of said electrical field being inversely proportional to said first and second distances.
 2. The apparatus as recited in claim 1 wherein a difference between said first distance and said second distance defines an electric field gradient, with a portion of said electric field present between said protrusion and said substrate being greater than a subsection of said electric field present between each of said pair of spaced-apart recessions and said substrate, with said portion having sufficient magnitude to create a contiguous region of said liquid on said an area of said substrate in superimposition with said protrusion.
 3. The apparatus as recited in claim 1 wherein said protrusion consists of Indium Tin Oxide (ITO).
 4. The apparatus as recited in claim 1 wherein said template further includes a layer of Indium Tin Oxide (ITO) and said pair of spaced-apart recessions and said protrusion are formed in said layer of ITO.
 5. The apparatus as recited in claim 1 wherein said template further includes a layer of fused silica and a layer of Indium Tin Oxide (ITO).
 6. The apparatus as recited in claim 1 wherein said template further includes a layer of fused silica and a layer of Indium Tin Oxide (ITO), with said source of voltage being in electrical communication with said layer of ITO.
 7. The apparatus as recited in claim 1 wherein said template is substantially transparent to ultraviolet light.
 8. The apparatus as recited in claim 1 wherein said template further includes a fluorine containing monolayer.
 9. The apparatus as recited in claim 1 wherein template further includes a layer of Indium Tin Oxide (ITO) and said pair of spaced-apart recessions and said protrusion are formed in said layer of ITO and further including a fluorine containing monolayer positioned adjacent to said layer of ITO, with said fluorine containing monolayer being positioned between said substrate and said layer of ITO.
 10. An apparatus for patterning a liquid on a substrate, said apparatus comprising: a template having a plurality of protrusions, each of which is spaced-apart from said substrate a first distance, and a plurality of recessions, each of which is spaced-apart from said substrate a second distance; and a source of voltage in electrical communication with said template to produce an electric field between said template and said substrate, with a difference between said first distance and said second distance defining a plurality of electric field gradients, with a portion of said electric field present between said plurality of protrusions and said substrate being greater than a subsection of said electric field present between said plurality of recessions and said substrate.
 11. The apparatus as recited in claim 10 wherein said plurality of protrusion consist of Indium Tin Oxide (ITO).
 12. The apparatus as recited in claim 10 wherein said template further includes a layer of Indium Tin Oxide (ITO), with said plurality of protrusions and said plurality of recessions being present in said layer of ITO.
 13. The apparatus as recited in claim 10 wherein said template further includes a layer of fused silica and a layer of Indium Tin Oxide (ITO).
 14. The apparatus as recited in claim 10 wherein said template further includes a layer of fused silica and a layer of Indium Tin Oxide (ITO), with said source of voltage being in electrical communication with said layer of ITO.
 15. The apparatus as recited in claim 10 wherein said template is substantially transparent to ultraviolet light.
 16. The apparatus as recited in claim 10 wherein said template further includes a fluorine containing monolayer.
 17. The apparatus as recited in claim 10 wherein said template further includes a layer of Indium Tin Oxide (ITO), with said plurality of recessions and said plurality of protrusions are formed in said layer of ITO and further including a fluorine containing monolayer positioned adjacent to said layer of ITO, with said fluorine containing monolayer being positioned between said substrate and said layer of ITO.
 18. The apparatus as recited in claim 10 wherein said portion of said electric field has sufficient magnitude to create a contiguous region of said liquid on an area of said substrate in superimposition with said plurality of protrusions.
 19. An apparatus for patterning a liquid on a substrate, said apparatus comprising: a template having a plurality of protrusions and recessions, spaced apart from said substrate, with said liquid being disposed therebetween; a source of voltage in electrical communication with said template to produce an electric field between said template and said substrate, with a subportion of said electric field present between each of said plurality of protrusions being greater than a subpart of said electric field present between each of said plurality of recessions, with adjacent subportions and subparts defining an electric field gradient, with said subportions having sufficient magnitude to move said liquid to form a continuous region of said liquid between each of said plurality of protrusions and said substrate, and said electric field gradient preventing said liquid from forming a continuous area of said liquid in regions of said substrate in superimposition with each of said plurality of recessions.
 20. The apparatus as recited in claim 19 wherein said template further includes a layer of Indium Tin Oxide (ITO), with said plurality of protrusions and recessions being present in said layer of ITO.
 21. The apparatus as recited in claim 19 wherein said template further includes a layer fused silica and a layer of Indium Tin Oxide (ITO).
 22. The apparatus as recited in claim 19 wherein said template further includes a layer of fused silica and a layer of Indium Tin Oxide (ITO), with said source 